Structural Properties of Weakly Segregated PS−PB Block Copolymer Micelles in n-Alkanes: Solvent Entropy Effects

Unimer suppression enables supersaturated homopolymer swollen micelles with long-term stability after glassy entrapment

Micelle sizes are critical for a range of applications where the simple ability to adjust and lock in specific stable sizes has remained largely elusive. While micelle swelling agents are well-known, their dynamic re-equilibration in solution implies limited stability. Here, a non-equilibrium processing sequence is studied where supersaturated homopolymer swelling is combined with glassy-core ("persistent") micelles. This path-dependent process was found to sensitively depend on unimer concentration as revealed by DLS, SAXS, and TEM analysis. Here, lower-selectivity solvent combinations led to the formation of unimer-homopolymer aggregates and eventual precipitation, reminiscent of anomalous micellization. In contrast, higher-selectivity solvents enabled supersaturated homopolymer loadings favored by rapid homopolymer insertion. The demonstrated ∼40-130 nm core-size tuning exceeded prior equilibrium demonstrations and subsequent core-vitrification enabled size persistence beyond 6 months. Lastly, the linear change in micelle diameter with homopolymer addition was found to correlate with a plateau in the interfacial area per copolymer chain.

Ionic Strength-Dependent Structure of Complex Coacervate Core Micelles

Salt concentration-dependent structure of complex coacervate core micelles (C3Ms), formed by polyether-based block copolyelectrolytes containing cationic ammonium (A) or anionic sulfonate (S) groups in aqueous media, is investigated by light scattering and small-angle X-ray/neutron scattering (SAX/NS). As the salt concentration increases, both a core radius (Rcore) and an aggregation number (Nagg) significantly decrease, but a corona thickness (Lcorona) is nearly unchanged. Larger salt concentrations can lower the interfacial tension between the coacervate cores and aqueous media, resulting in an increased interfacial area per chain and a more relaxed conformation of the core blocks. Based on the structure characterization, the scaling relationship between structure parameters (i.e., Rcore, Nagg, and Lcorona) and salt concentration is obtained and compared to the theoretical description estimated by the free energy balance between the entropic penalty of core stretching and the interfacial energy. We propose that the free energy contribution of the core block stretching is not negligible in C3Ms because of the highly swollen cores caused by water.

Determination of Reaction Kinetics by Time-Resolved Small-Angle X-ray Scattering during Polymerization-Induced Self-Assembly: Direct Evidence for Monomer-Swollen Nanoparticles

The kinetics of heterogeneous polymerization is determined directly using small-angle X-ray scattering (SAXS). This important advancement is exemplified for the synthesis of sterically-stabilized diblock copolymer nanoparticles by reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate (BzMA) in mineral oil at 90 °C. The principle of mass balance is invoked to derive a series of equations for the analysis of the resulting time-resolved SAXS patterns. Importantly, there is a continuous change in the X-ray scattering length density for the various components within the reaction mixture. This enables the volume fraction of unreacted BzMA monomer to be calculated at any given time point, which enables the polymerization kinetics to be monitored in situ directly without relying on supplementary characterization techniques. Moreover, SAXS enables the local concentration of both monomer and solvent within the growing swollen nanoparticles to be determined during the polymerization. Data analysis reveals that the instantaneous rate of BzMA polymerization is proportional to the local monomer concentration within the nanoparticles. In principle, this powerful new time-resolved SAXS approach can be applicable to other heterogeneous polymerization formulations.

Self-Assembly of Surfactin into Nanofibers with Hydrophilic Channels in Nonpolar Organic Media

We investigated the self-assembly of surfactin (SFNa), a cyclic peptide amphiphile produced by Bacillus subtilis, in a nonpolar organic solvent, namely, cyclohexane (CHx). The CHx solution of SFNa formed a thermoreversible organogel. Transmission electron microscopy and small-angle X-ray scattering (SAXS) analyses showed that gelation of the CHx solution of SFNa was caused by physical cross-linking of SFNa nanofibers. Wide-angle X-ray diffraction and Fourier-transform infrared analyses showed that the SFNa nanofibers were formed by one-dimensional stacking of SFNa rings with a period of 0.48 nm corresponding to the length of inter-ring hydrogen bonds between amide groups. A combination of SAXS and small-angle neutron scattering investigations of CHx and deuterated CHx solutions of SFNa nanofibers containing H₂O or D₂O showed that the SFNa nanofibers had a hydrophilic interior and formed water channels by water incorporation in this region.

Supramolecularly cross-linked amphiphilic block copolymer assembly by the dipolar interaction of a merocyanine dye

Dipolar interaction driven dimerization of a merocyanine (MC) dye has been exploited to achieve non-covalently crosslinked stable micelles in water and reverse micelles in toluene with emissive properties from a MC-pendant amphiphilic block copolymer.

Self-Assembly of Block and Graft Copolymers in Organic Solvents: An Overview of Recent Advances

This review is an attempt to update the recent advances in the self-assembly of amphiphilic block and graft copolymers. Their micellization behavior is highlighted for linear AB, ABC triblock terpolymers, and graft structures in non-aqueous selective polar and non-polar solvents, including solvent mixtures and ionic liquids. The micellar characteristics, such as particle size, aggregation number, and morphology, are examined as a function of the copolymers' architecture and molecular characteristics.

Micelle response to changes in solvent properties

The dynamics of co-polymer systems play an important role in the preparation and stability of formulations, as well as on their capability to function in drug delivery systems. Micelle inversion can occur as a result of a change in concentration when a solvent is very volatile and evaporates, or as a result of a change in solvent quality upon addition of another solvent to the original solution, or upon changes in pH. In this work, dissipative particle dynamics (DPD) is used to examine the dynamics of micelle inversion in concentrated systems of diblock and triblock amphiphiles, where interactions between neighboring aggregates are observed. Significant differences were observed in the inversion process of different amphiphilic molecules, with a large amount of co-polymer exchange between inverting aggregates made of diblock amphiphiles, and practically no exchange of molecules between aggregates during the inversion of triblock copolymer aggregates. Fundamental mechanisms of inversion are revealed that provide information which can be used to help design micelles for targeted drug release and allow understanding of history dependant formulations.

Dielectric discontinuity in equilibrium block copolymer micelles

The surface tension between the hydrophobic core and the solvent is known to play a major role in the self-assembly of diblock copolymer micelles in solution. Coulombic forces are also very important in the case of aggregates with weakly charged coronas. The aggregation number and morphology are often tuned by the addition of electrolytes to the solution via electrostatic screening and an eventual change in solvent quality. However, when the surface tension is low enough, dielectric discontinuity between the core and the solvent becomes important enough in comparison to other mechanisms, driving the surface tension at the same time it screens electrostatic interactions in the corona. Below, we demonstrate the importance of this effect for micelles with neutral and weakly charged coronas.

Study of the internal structure of polymer micelles by anomalous small-angle X-ray scattering at two edges

Anomalous small-angle X-ray scattering with two marker elements was applied to the structural analysis of poly(4-vinylphenol rubidium salt)-block-poly(4-bromostyrene) (RbPVPh-b-PBrS) micelles, where Br and Rb were the markers for the hydrophobic core and the hydrated corona, respectively. By using two different markers for the hydrophobic core and the hydrated corona, the form factors of the core and corona were extracted separately from the scattering profile of the whole RbPVPh-b-PBrS micelles. The form factor of the hydrophobic core (the spatial distribution of Br) revealed that the core was regarded as a solid sphere with a smooth surface and a radius of 47 nm. Conversely, the form factor of the spatial distribution of Rb+indicated that the shell of the RbPVPh-b-PBrS micelles was 15 nm thick.

Chain Exchange in Binary Copolymer Micelles at Equilibrium: Confirmation of the Independent Chain Hypothesis

The mechanism of chain exchange in block polymer micelles at equilibrium is investigated using time-resolved small-angle neutron scattering (TR-SANS). The binary micelles are formed from blends of two poly(styrene-b-ethylene-alt-propylene) (PS-PEP) copolymers with different PS core block lengths, only one of which is contrast-matched with the solvent, squalane, so that the monitored scattering intensity only reflects the other species. Micelles prepared with an excess of deuterated PS chains (of the visible species) and those with the equivalent protonated PS chains are blended ("postmixed") at room temperature, where the exchange of chains is suppressed. At several elevated temperatures these samples were monitored by TR-SANS, in which mixing of isotope-labeled visible species gives systematic reduction of scattering signals with time and provides a quantitative way to characterize the micelle exchange kinetics. Within experimental error, the results for each labeled chain (i.e., longer or shorter) in the binary micelles are identical to those recently reported for the same labeled chains in the corresponding single block copolymer component micelles, thus proving that chain exchange in these micelles involves independent chain motion. This reinforces the important conclusions that the single-chain exchange mechanism dominates in the studied micelle solutions and that micelle fusion or fission events are rare.

Molecular Exchange in Diblock Copolymer Micelles: Bimodal Distribution in Core-Block Molecular Weights

The role of core block size dispersity on the rate of molecular exchange in spherical micelles formed from 1% by volume poly(styrene-b-ethylenepropylene) (PS-PEP) diblock copolymers in squalane (C₃₀H₆₂) was investigated using time-resolved small-angle neutron scattering (TR-SANS). Separate copolymer solutions (total polymer 1% by volume) containing either deuterium labeled (dPS) or normal (hPS) poly(styrene) core blocks were prepared and mixed at room temperature, below the core glass transition temperature. Each preparation (dPS or hPS) contained equal volume fractions of M_n = 26 and 42 kg/mol (h-equivalent) poly(styrene) blocks. Heating to temperatures between 87 and 146 °C resulted in block copolymer exchange as evidenced by a systematic reduction in the SANS intensity; C₃₀H₆₂ and C₃₀D₆₂ were blended so as to contrast match the fully exchanged cores. Following a protocol established in a previous report, the time-dependent intensity data were shifted with respect to time and temperature, leading to a master curve covering nearly 7 orders of magnitude in reduced time. These results are quantitatively accounted for by summing the weighted relaxation functions obtained from the individual components, consistent with a previously published model that accounts for the dramatic sensitivity of the molecular exchange dynamics to core block dispersity.